Offscreen

When you're watching a movie or playing a game on a screen, you can see what's going on in front of you - that's the onscreen part. But sometimes there are things happening that you can't see, like when a character is standing behind a wall or a building that's not shown on the screen. That's the offscreen part. It's like when you're playing hide and seek and you can't see where your friend is hiding - they're offscreen. So, offscreen basically means things that are happening or existing outside of what you can directly see on the screen or in front of you.